Molecular aggregates have garnered significant interest due to their unique photophysical properties stemming from coulombic and vibrational coupling between molecules, facilitating enhanced exciton transport. Kasha's rule categorizes aggregates into H-aggregates (parallel transition dipoles, hypsochromic shift) and J-aggregates (head-to-tail dipoles, bathochromic shift). The formation of aggregates is largely governed by intermolecular interactions, including hydrogen bonding, which plays a crucial role in self-assembly. Hydrogen bonding has been observed in various systems, such as DNA and organic semiconductors like indigo, significantly influencing their optical and electronic properties. Oleylamine, a commonly used capping ligand in nanomaterial synthesis, possesses amine groups capable of forming hydrogen bonds. This study investigates the self-assembly behavior of an oleylamine-acetone system, exploring the potential of hydrogen bond-mediated aggregation to improve charge transport in QD-based devices. The challenge of surfactant molecules hindering charge transport in QD devices necessitates novel approaches, such as those described herein, for improving device performance.

Extensive research has explored the properties of molecular aggregates, focusing on exciton transport and the impact of intermolecular interactions. Kasha's seminal work laid the foundation for understanding the photophysics of molecular aggregates, classifying them into H- and J-aggregates based on the arrangement of transition dipoles. Subsequent research investigated the effects of vibronic coupling and charge transfer on these aggregates. Studies highlighted the role of various intermolecular interactions like hydrogen bonding, π-π stacking, and halogen bonding in driving self-assembly and influencing aggregate structure and optical properties. Hydrogen bonding, in particular, has been shown to enhance delocalization effects in organic semiconductors. The use of oleylamine as a capping ligand in nanomaterial synthesis is well-established, but its role in influencing charge transport within devices is an area of ongoing investigation. The potential for controlled aggregation of oleylamine and its impact on device properties has not been previously explored in detail.

The experimental section involved mixing oleylamine and acetone in varying ratios. Two distinct scenarios were investigated: (1) Immiscible Oleylamine-Acetone system with low acetone concentration (5 mL oleylamine and 2 mL acetone). (2) Miscible Oleylamine-Acetone system with high acetone concentration (5 mL oleylamine and 10 mL acetone). These mixtures were aged for several weeks, observing changes in their optical properties (absorption and emission spectra). Different dilutions of the aged samples were also analyzed to examine the reversibility of the emission changes. For In₂S₃ QD preparation, indium chloride and elemental sulfur were used as precursors, while dodecanethiol and oleylamine served as capping agents. The hot injection technique was employed to synthesize In₂S₃ QDs. These QDs were then subjected to an acetone-assisted aging process. Optical characterization techniques such as UV-Vis spectroscopy, photoluminescence spectroscopy, and time-correlated single photon counting (TCSPC) were used to investigate the optical properties. High-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray diffraction (XRD) were employed for structural characterization. For the photocurrent device, a sandwich structure was fabricated using the In₂S₃ QD-oleylamine-acetone hybrid system as the active layer, along with TiO₂, MoO₃, and Al as the supporting layers. The I-V characteristics of the devices were measured under dark and illumination conditions. Quantum chemical calculations using density functional theory (DFT) were used to study the interactions between oleylamine and acetone molecules.

Aging the oleylamine-acetone mixture resulted in a significant red shift in both absorption and emission maxima, indicative of aggregation. The emission color changed from blue to orange or red. HRTEM revealed the formation of nanoscale molecular clusters self-assembling into flower-like aggregates. FTIR analysis confirmed the presence of hydrogen bonds between the oleylamine and acetone molecules, evident by a red shift in the carbonyl (C=O) stretching frequency. Raman spectroscopy showed a broadening of Raman bands upon aging, consistent with the formation of molecular clusters. The emission wavelength exhibited reversible tunability, shifting from red to blue (and vice-versa) by controlling the dilution of the aged samples. This tunability indicates a dynamic equilibrium between the nanoscale clusters and the larger flower-like aggregates. Time-resolved measurements showed that the photoexcited charge carriers in the aged sample undergo faster relaxation compared to the un-aged sample. The ratio of oleylamine to acetone is critical in determining the reaction pathway. At a higher acetone concentration, the formation of hydrogen-bonded oleylamine-acetone molecular clusters is favored. Lower acetone concentration leads to imine bond formation between oleylamine and acetone, producing oleylimine. Theoretical calculations confirmed that the strongest interactions occurred between the oleylamine and acetone molecules, supporting the experimental observations. In the In₂S₃ QD system, aging resulted in the removal of oleylamine from the QD surface, increasing the QD size. The In₂S₃ QDs became embedded in the oleylamine-acetone aggregate matrix. Photocurrent devices fabricated using the In₂S₃ QD-oleylamine-acetone aggregate hybrid system exhibited a remarkable enhancement in photocurrent compared to devices made with un-aged samples.

The results demonstrate that aging-induced aggregation of an oleylamine-acetone system leads to significant changes in its emission properties. This is attributed to the formation of hydrogen-bonded nanoscale clusters which then further aggregate into larger structures. The reversible tuning of emission color through dilution suggests a dynamic equilibrium between cluster and aggregate states, with the cluster state exhibiting blue emission and the aggregated state exhibiting red emission. The successful integration of the aged oleylamine-acetone system into In₂S₃ QD devices resulted in a dramatic increase in photocurrent. This is explained by the removal of insulating oleylamine from the QD surface, along with the formation of a conducting molecular aggregate matrix that facilitates enhanced charge transport. The findings highlight a novel approach for improving charge transport in QD-based devices by strategically controlling the surface passivation and forming a conducting matrix. The enhanced photocurrent observed underscores the potential of this method for developing high-performance optoelectronic devices.

This study successfully demonstrated the formation of hydrogen-bond mediated molecular clusters and aggregates in an oleylamine-acetone system, resulting in a tunable emission behavior. The critical role of the oleylamine-to-acetone ratio in determining either hydrogen-bonding or imine-bond formation was established. The application of this aging-assisted process to In₂S₃ QDs significantly enhanced the photocurrent generation in a fabricated device. This work opens up avenues for improving the performance of QD-based devices by manipulating the surface ligands and creating conducting aggregate matrices. Future research could explore the use of this approach with other QD systems and investigate the detailed charge transfer mechanisms within the hybrid system.

The study primarily focused on the oleylamine-acetone system and its interaction with In₂S₃ QDs. Further research is needed to explore the generality of this approach with other surfactant molecules and QD systems. The exact mechanism of charge transport within the aggregate network warrants further investigation. The long-term stability of the aged In₂S₃ QD-aggregate hybrid system under device operating conditions requires additional study. The theoretical calculations were limited to exploring a selected set of Oleylamine-Acetone complexes. A more comprehensive theoretical study involving a broader range of configurations would provide deeper insights.